1. Field of the Invention
The present invention relates to a low-noise junction field effect depletion mode transistor amplifier and circuitry for retrieving the amplified signal therefrom. More specifically, the present invention relates to a low-noise junction field effect transistor amplifier for amplifying a signal generated by the piezoelectric transducer of a bone conduction microphone.
2. Description of the Prior Art
Bone conduction microphones for transmitting speech communications are generally known in the prior art. These microphones generally comprise a piezoelectric crystal which is attached to a bony portion of the human body such as the top of the head or skull. These microphones offer the advantage that the speech intelligence to be transmitted is transmitted directly through the bone of the speaker to the piezoelectric crystal of the microphone and therefore does not pick up any ambient noise or sound such as would be picked up by a conventional microphone held in front of the speakers mouth. Examples of such prior art bone conduction microphones can be found in U.S. Pat. No. 3,723,670 to Sebesta et al; U.S. Pat. No. 3,787,641 to Santori; and U.S. Pat. No. 2,255,250 to Greibach; and U.S. Pat. No. 3,087,028 to Bonnin.
In each of the above bone conduction microphones there is a need for a high gain low-noise solid state amplifier which can be contained in a compact arrangement within the housing of the microphone per se. It is generally known that field effect transistors (FET) function relatively well as active amplifying elements for the output signal of a piezoelectric transducer. However, heretofore the conventional FET amplifier arrangements have not possessed sufficiently low noise levels to operate effectively as the amplifier of a bone conduction microphone.